No matter what you'll have code on their servers. Some languages have obfuscation/encryption you can use. Overall from a tech perspective, I'd suggest building

No matter what you'll have code on their servers. Some languages have obfuscation/encryption you can use. Overall from a tech perspective, I'd suggest building the app into containers (docker), so the computer they use at least becomes less important. Not sure what that means for users of Windows. (Docker won't protect against code theft but is a nice abstraction so the host OS matters less)